1. Field of the Invention
The present invention relates generally to optical memory, and more particularly, to holographic memory which utilizes the Talbot Effect for lensless imaging of periodic structures to map periodic phase structures at one plane in a holographic memory system to the input data plane in the system by propagating a lenslet array to self-image coincident on a spatial light modulator which provides the holographic memory system with input data.
2. Description of the Prior Art
Holographic memory systems store vast amounts of data. This may be useful for archival, read-only applications, or in an active memory system. The basic principle behind these systems involves arranging data in pages, which are rectangular images, and recording these pages on holograms. When the data is to be retrieved, the appropriate page is reconstructed from the hologram.
Most modern processing systems, including personal computers (PCS), rely on one form or another of optical data storage. For example, CD-ROM drives are now standard equipment on nearly all new PCS. Nearly all multimedia software, including video games, maps, encyclopedias, and the like, are sold on CD-ROM. Also, compact discs are the most prevalent storage medium for musical recording. More recently, digital video disc (DVD) technology has been introduced that will expand the storage capacity of standard CD technology from about one-half gigabyte to about five gigabytes.
The large storage capacities and relatively low costs of CD-ROMs and DVDs have created an even greater demand for still larger and cheaper optical storage media. Many large businesses rely on jukebox-style CD changers in order to access a particular one of potentially hundreds of discs. Motion pictures released in optical storage format still require multiple CDS, DVDs or oversized laser discs. However, it appears that the limits of CD-ROM and DVD technology are being reached. In order to continue to improve the capacity and speed of optical storage systems, research increasingly focuses on holographic storage devices capable of storing hundreds of gigabytes in a CD-sized storage medium.
A number of holographic data storage systems have been developed that are capable of storing and retrieving an entire page of data at a time. In these systems, data to be stored is first encoded in a two dimensional (2D) optical array, for example on a liquid crystal display (LCD) screen, which is one type of spatial light modulator (SLM). Another type of SLM is Texas Instruments' Digital Mirror Device, which is a reflective device that allows the reflectivity of each pixel to be changed. The term "SLM" also includes fixed masks of varying optical density, phase, or reflectivity.
A first laser beam, a plane wave, is transmitted through the SLM and picks up an intensity and/or phase pattern from the data squares and rectangles (pixels) in the 2D array. This data-encoded beam, called an object beam, is ultimately projected onto and into a light-sensitive material, called a holographic memory cell (HMC). A second laser beam, called a reference beam, is also projected onto and into the holographic memory cell. The object beam and the reference beam then cross at the HMC to produce an interference pattern throughout a volume element of the HMC. This unique interference pattern induces material alterations in the HMC that generate a hologram.
The formation of the hologram in the holographic memory cell is a function of the relative amplitudes and polarization states of, and the phase differences between, the object beam and the reference beam. It is also highly dependent on the incident angles at which the object beam and the reference beam were projected onto the holographic memory cell. After hologram storage, the data beam may be reconstructed by projecting into the HMC a reference beam that is the same as the reference beam that produced the hologram. The hologram and the reference beam then interact to reproduce the data-encoded object beam, which may then be projected onto a two-dimensional array of light sensitive detectors which read back the data by sensing the pattern of light and dark pixels.
In holographic memory systems, it is often advantageous to phase encode the input amplitude data structure. Such phase encoding redistributes the amplitude data structure's Fourier transform pattern into a distribution that is better suited for holographic memory systems. However, the selected phase encoding structure must be made optically coincident with the amplitude encoding structure for optimal performance. Such coincidence is generally achieved by four-f imaging the phase structure onto the amplitude structure, or vice versa. The addition of a complete four-f imaging system in order to accomplish such coincidence has inherent drawbacks in system size, cost, complexity, and weight.
Of the infinite choices for such phase encoding structures, some could be periodic in nature. While it is often undesirable to utilize most periodic phase structures in holographic memory systems (because doing so results in little benefit in the Fourier plane restructuring), some may be desirable. One class of periodic phase structures that could be advantageous to use in phase altering the amplitude pattern of the input data set in a holographic memory system is a lenslet array. Use of a lenslet array acts to collapse the higher Fourier order energy into the zero order, while dispersing the D.C. part of the orders locally.
U.S. Pat. No. 5,859,808, assigned to the assignee of the present application, discloses Systems and methods for steering an optical path to gain access to data locations in a holographic memory cell. One of the systems includes: (1) a refractive element that receives a complex, spatially-modulated incident beam of light, (2) first and second reflective elements locatable to receive and reflect the incident beam in a Fresnel region thereof and (3) a reflective element steering mechanism, coupled to the first and second reflective elements, that moves the first and second reflective elements in tandem to steer the incident beam with respect to the HMC thereby to cause the incident beam to illuminate a location on the HMC that is a function of a movement of the first and second reflective elements.
U.S. Pat. No. 4,813,762 entitled "Coherent Beam Combining of Lasers Using Microlenses and Diffractive Coupling" discloses a diffractive lenslet Array which receives light from multiple lasers. The lenslet array is spaced apart from a partially reflecting mirror by a distance Z=nd.sup.2 /.lambda., where n is an integer or half integer, .lambda. is the laser wavelength and d is the spacing of the lenslets in the array. In a preferred embodiment of the '762 Patent, the apparatus is a unitary design in which the lenslets are etched into one surface of a substrate and a parallel surface is coated to form the partially reflecting mirror. The lenslets abut one another to produce a fill factor (percentage of array containing light) close to one and each of the lenslets is a multi step diffractive lens. Diffractive spreading over a round trip distance from lasers to mirror and back again causes feedback light from a single lenslet to couple into adjacent lenslets. The light from all the lenslets is coupled back into the laser waveguides efficiently only when the wave front at each of the lenslets is flat, that is, when the phase of the feedback is uniform across a lenslet. Uniformity is achieved when the separation between lenslet array and mirror is the Talbot self-imaging condition set forth above.